1. Field of the Invention
The present invention relates, in general, to industrial machine control, and, more particularly, to a system and method for sensing position of an overhead crane for process automation and collision avoidance.
2. Relevant Background
Motorized vehicles are commonly used in industrial environments such as manufacturing facilities, assembly facilities, and warehouses. For example, an industrial facility may include one or more overhead cranes that are used to transport heavy items or materials from one location to another.
A typical overhead crane includes a pair of rails spanning a distance across the facility and a bridge that spans the rails. The bridge is motorized to move along an axis defined by the rails and may include a tram that moves along the bridge between the rails. The tram may be controlled manually through a control box that is coupled by wires or radio control device. Alternatively, an automated factory uses programmable computers to control and coordinate the actions of various pieces of machinery in the factory.
With either manual or automated control, it is important to track the position of the motorized vehicle with accuracy and precision. To ease manual control and enable automated control, position information is required to pick up and deliver payloads as required. Moreover, overhead cranes are relied on to move heavy loads quickly but to minimize swinging and pendulum action of the load as the crane stops and starts. Hence, the crane must speed up and slow down based upon knowledge of its current position and the desired destination.
A further use of position information is to avoid collisions between the motorized vehicle and other objects such as factory walls or other motorized vehicles operating in the same work space. Collision avoidance is readily implemented if reliable, precise position information is available for all of the motorized equipment in a factory.
Prior position sensing equipment used ultrasonic or radio frequency devices (i.e., RADAR). Ultrasonic devices are subject to multiple echoes and reflections in an industrial environment. These echoes and reflections make it difficult to obtain accurate and reliable position information. Radio frequency systems exhibit fewer problems with reflections and echoes, but are susceptible to radio frequency interference (RFI). RFI is a significant noise problem in an industrial environment in which large, high speed motors and welding equipment, for example, are in use. What is needed is an industrial position sensor that is immune to echoes, undesired reflections, and RFI.
Because of their accuracy and RFI noise immunity, there have been attempts to use lasers for position sensing equipment. Such systems mount a laser and a photodetector on the motorized equipment. Both the laser and photodetector are initially aligned to a reference object positioned along the axis of travel for the motorized equipment. To sense position, the laser is activated and the photodetector generates a signal from the energy reflected by the reference object. The time between LASER activation and reception of the reflected signal is measured and indicates distance between the motorized vehicle and the reference object. The reference object may be fixed or movable depending on whether fixed or relative position information is desired.
Unfortunately, laser devices capable of sufficient power output to work as position sensors are costly. Also, lasers output a characteristically narrow beam that does not diverge significantly as it travels through space. While non-divergence is a desirable property from a power efficiency standpoint, it makes alignment of a laser with a reference object problematic. Moreover, industrial cranes, for example, may bend slightly in the middle of their span when carrying heavy loads. This bending can cause a position sensor mounted on the motorized vehicle to skew several degrees from its position at the ends of the span. This skewing is not a problem for ultrasonic or radio frequency position sensors that use wide beam width signals for sensing. However, the narrow beam width of lasers results in failure of the laser beam to reach the reference object and/or successfully reflect back to the position sensor for a measurement. A need remains for an industrial position sensor with noise immunity and precision similar to laser systems but that is easily aligned and immune to misalignment caused by crane skewing.
Briefly stated, the present invention involves a sensor for determining the position of a movable object along a selected axis. The system includes a target positioned at a location aligned with the selected axis. An optical energy emitter is mounted on the movable object and has a beam dispersion greater than two degrees directed at the target. An optical energy receiver is mounted on the movable object and aligned to receive optical energy reflected by the target. The optical energy detector generates a receive signal indicating reception of the optical energy. A time of flight circuit coupled to the emitter and receiver generates a flight time signal indicating the elapsed time from emission of the optical energy to reception of reflected optical energy. A control circuit monitors the flight time signal and outputs a position signal indicating position of the movable object with respect to the target.
In another aspect, the present invention involves a method for operating a spatial positioning system for determining the linear position of a movable object along a selected axis. A reflective target is mounted at a location on the selected axis and an optical energy emitter is adjustably mounted on the movable target. An optical energy receiver is adjustably mounted on the movable object and the emitter and receiver are coarsely aligned with the target. The power received by the optical energy receiver is monitored while adjusting the alignment. The emitter and receiver are rigidly affixed to the movable object when the power received is above a preselected threshold.
In still another aspect, the present invention involves a precision digital-to-analog converter driving the output of a spatial positioning system. A first pulse width modulation (PWM) generator generates a first N-bit PWM signal on an output. A second PWM generator generates a second M-bit PWM signal on an output. A first resistor has a first end coupled to the output of the first PWM generator. A second resistor has a first end coupled to the output of the second PWM generator. The first and second resistor are chosen such that a ratio of the first resistor to the second resistor is 1:2N. A summing node couples the second ends of the first and second resistors. A low pass filter removes the high frequency components from the combined PWM signal to generate an analog output.